We tested the inhibition of the whole cell currents in primary GBM and U-87 MG cells at 23 nM IbTx concentration which is ~2–10-fold the IC50 for KCa1.1 inhibition25 and compared the RCF to that determined using 1 μM paxilline, which is expected to block fully the KCa1.1 current when applied at negative holding potentials (IC50~12 nM). This evidence concerns the gene KCNMA1 and glioblastoma.